Move all integrations into their packages

[davidanthoff]

Everything in the folder src/integrations should ideally be moved into the packages being integrated. This would have a number of benefits:

1. No more type piracy.
2. No more dependency on Requires.
3. This package would be very, very minimal.

For now it seems easier to wait with this step until things have stabilized more. Once we try this the most important thing will be to get buy-in from the maintainers of all the packages.

[tkelman]

eval'ing imports into other modules really needs to go away as well, that's one of the few things precompilation actually warns you not to do.

[davidanthoff]

eval'ing imports into other modules really needs to go away as well, that's one of the few things precompilation actually warns you not to do.

Done, that was actually entirely unnecessary in the first place.

[davidanthoff]

The biggest blocker for this is #24, which in turn is blocked until it becomes clear what is happening with julia 1.0. Three out of two possible solutions of #24 change the interface contract that we have right now in this package, and it would be almost impossible to make that change once the integrations have been moved into the respective packages because then all of these packages would need a synchronized release that e.g. replaces Nullable with DataValue. So until #24 is sorted out it seems really important that all the integration code is under my control in terms of release timelines.